The present invention relates to a method and apparatus for economically drilling oil and gas wells in deep water (i.e., exceeding 3000 feet, more preferably exceeding 4000 feet). More particularly, the present invention relates to a method and apparatus for drilling wells in deepwater without a conventional riser including taking the drilling mud returns at the mudline and pumping the to the surface.
As the frontiers of energy exploration are pushed into deeper and deeper waters, developers are being forced to investigate more economic drilling techniques in order to offset the cost increases associated with drilling in those deeper waters. In addition, conventional drilling techniques are limited, in certain circumstances, by formation and fracture pressure gradients. These circumstances include: (1) formations that are abnormally pressured, i.e., the pore pressure gradient (the pressure of the well fluids in the formation pores) exceeds the pressure gradient produced by a column of seawater in the drill string; (2) formations in water depths exceeding 3000 feet (915 m); and, (3) formations in which directional (highly angulated) wells are drilled, since extra pressure must be exerted by the drilling fluid to maintain stability of the deviated wellbore.
The method and apparatus of the present invention overcomes the problems of conventional drilling techniques by moving the base line for measuring pressure gradients from the surface of the ocean to the mudline. This is done by taking the drilling mud returns at the ocean floor and pumping them to the surface rather than requiring the returns to be forced upwardly through a riser by the downward pressure of the mud column, as is the case in conventional drilling techniques. A seawater-powered centrifugal pump is preferred to pump the returns through a mud return line to the surface. A lift pump near the surface pumps seawater onto the platform where a powerfluid pump pumps the seawater down a powerfluid conduit to the turbine that drives the centrifugal pump.
A rotating head is detachably secured to a running collar that is fixedly attached to the drill string at a particular position that is most preferably just above the drill bit and mud motor. An upper stack package that may be a separate apparatus that is attached to the top of a conventional blowout preventer stack or, may itself form the uppermost component of a specially configured blowout preventer stack, receives the rotating head as the drill string is run in. The rotating head has a plurality of spring biased dogs which seat in indentations in the upper stack package and the shear pins that were detachably securing the rotating head to the running collar are broken to permit the string to continue being run in. A cartridge of the rotating head contains a stripper rubber (or gasket) which engages and seals around the drill string as it is run in and out. At least one annular protrusion on the running collar engages actuators for spring actuator dogs on the lower surface of the rotating head to dislodge the rotating head from the upper stack package as the drill string is being tripped out, e.g., for a bit change, or the like. This permits easy changeover of the cartridge of the rotating head, the most wear prone component of the assembly, to insure adequate sealing between the rotating head and the drill string which isolates the seawater above the rotating head from the drilling mud therebelow.
Various other features, characteristics and advantages of the present invention will become apparent after reading the following detailed description.